1. Field of the Invention
The invention relates to a multilayer film comprising at least one layer of plasticized polyvinyl acetal with reduced flowability.
2. Description of the Related Art
Plasticized polyvinyl butyral has long been known as an intermediate layer for laminated safety glasses for architectural and automobile application as well as an adhesive film for photovoltaic modules.
In order to provide laminated safety glasses with sound damping properties, it is known to provide interlayer films comprising at least two layers of plasticized polyvinyl butyral having different mechanical properties. The different mechanical properties of the layers dissipate the mechanical energy of the sound waves which leads to sound damping. The different layers are provided with different mechanical properties by using different amounts or types of plasticisers and/or different polyvinyl butyrals.
In order to adjust the mechanical or physical properties of polyvinyl acetals, the molecular structure of its polymer chain was studied with regard to composition (degree of acetalisation, residual acetate content and residual alcohol content), chain length, aldehyde used and compatibility with different plasticizers.
Further studies were made to adjust the physical properties of polyvinyl acetals in view of the sequence of the repeating units of the polymer chain, i.e. acetal unit, acetate units and especially vinyl alcohol units.
For example, GB 2007677 discloses a process for the manufacture of polyvinyl acetals in which the sequence of repeating units in the polyvinyl acetal chain is “frozen” as not being in thermodynamic equilibrium. For this purpose, emulsifiers are used in the acetalisation reaction, which affects the sequence of groups in the polymer chain.
The use of emulsifiers to affect the polymer chain is furthermore disclosed in U.S. Pat. Nos. 8,053,504 and 4,970,245. In the process according to U.S. Pat. No. 4,970,245, low acetalisation temperatures are utilized to prevent the sequence of repeating units in the polyvinyl acetal from being in thermodynamic equilibrium.
On the contrary, U.S. Pat. No. 8,053,504 describes only acetalisation temperatures of at least 80° C. to control the polymer sequence. The sequence of repeating units of a polyvinyl acetals produced under such conditions (i.e. high reaction temperatures) are in thermodynamic equilibrium.
JP 3036894 discloses a process for production of polyvinyl butyral having a degree of acetalisation of 65-75 mol %, which comprises a two-step addition of acid catalyst and a reaction temperature of at least 69° C. The resulting polyvinyl butyral is characterized by the respective amount of heterotactic, syndiotactic and isotactic triades of hydroxyl groups. The polyvinyl acetal disclosed in JP 3036894 has an enhanced flowability, which is attributed to the amount of hydroxyl groups grouped in 3, 4 and 5 membered chains and the stereochemistry of the 3-membered chains.
In another example for enhancing flowability of polyvinyl acetal and plasticized films thereof, US20110155205A1 discloses polyvinyl acetal with high flowability and plasticizer-containing films made thereof. High flowability results in a viscosity (<75 mPas) of the polyvinyl acetal and high melt flow rate MFR of the plasticized polyvinyl acetal (i.e. the film) with MFR(100) of more than 500 mg/10 min. Such plasticized polyvinyl acetals or films have a reduced mechanical stability and are difficult to coextrude with plasticized polyvinyl acetal mixtures having different mechanical stability (i.e. melt flow rates MFR). Without being bound to any theory, this reason for the high flowability of polymers described in US20110155205A is the low concentration of acidic catalyst utilized in the acetalization process.
The prior art discussed above is concerned with enhancing the flowability of polyvinyl acetals, i.e. to achieve higher flowability. While this is beneficial for certain applications, a reduced flowability is desired in other circumstances. For example, reduced flowability would result in plasticized films made from such material with more rigid behaviour and/or improved mechanical stability.
An improved mechanical stability is beneficial for example for use in multilayer interlayer materials where layers of different flowability are combined. At the interface of multilayer films with the different layers of different/too high flowability, excessive air that is trapped in the laminate is likely to agglomerate at the interface and lead to the formation of bubbles and so called iceflowers.
Also, in multilayer laminates with one layer having a flowability which is too high, upon increased temperature, time and/or load, one glass pane may slip off in the case when both glass panes are not fixed. This so called creep-effect then leads to a displacement between a fixed back pane and a loose front glass pane.
In this regard, EP2660216A1 discloses multilayer films wherein the layers comprise chemically different polyvinyl acetals, thereby giving the layers different mechanical stability which has an impact on sound damping properties. However, EP2660216A1 is silent on the secondary structure of the polyvinyl acetal and the resulting mechanical properties as expressed by, for example, viscosity and melt flow rates MFR.
The object of the present invention was therefore to provide polyvinyl acetals with reduced flowability for producing layers for multilayer plasticizer-containing films.